It wasn't until the late nineteen-nineties that Portugal started the construction and marking of hiking paths. Long distance hiking trails are still rare. The major Grande Rota (GR's) are in part no more than a promiss. Of European long distance hiking trail E7 (GR12), 80 km on the border with Spain are ready. The GR11, Camino de Santiago, by now runs from Lisbon to Santiago de Compostela. Most fully completed is the 540 km long GR22, Trail of the Old Towns. Problematic in Portugal are the often outdated maps and sometimes poor condition of paths and markings.
As a good alternative you can hike a range of shorter trails, which you will come across in all national parks and protected landscape areas. These walks last from several hours to a full day, and are marked in red and yellow. Names of these trails start with PR, Pequena Rota (= minor routes), signposts mark the trailheads, although some trails are circular. Try to buy maps and guides through the internet or in you own country in advance, because locally these are hard to come by.
Walking the shorter trails is an excellent way to explore Portugese nature. The first national park that deserves to be mentioned is Peneda-Gerês, a mountain area abounding in water, in the north of Portugal forming the border with Spain. Another good area for hiking is the Serra de Estrela, where you will find mainland Portugal's highest mountain Malhão de Estrela (1993m). Most hikers will be looking for walks in the Algarve, and on the vulcanic islands of Madeira, the Azores (with Mount Pico as the highest at 2351m) and Berlengas. These popular islands harbour bizarre plant and animal life, and offer numerous short marked trails.